A screw compressor is one in which male and female screw rotors are disposed in a working chamber which is closely toleranced to the outside dimensions of the meshed pair of rotors. A suction port is formed at one end of the working chamber and a discharge port at the other. The suction and discharge ports of a screw compressor are essentially valveless openings which communicate with the working chamber of the compressor at its low pressure and high pressure ends respectively. The male and female rotors each have a high pressure end face which is parallel and closely proximate to the high pressure end wall of the working chamber.
In operation, low pressure gas enters the suction port and fills a chevron shaped chamber which is defined between the male and female rotors and by the walls of the working chamber. As the rotors rotate and mesh, the volume of the chevron shaped chamber decreases and the chamber is both axially and circumferentially displaced toward the high pressure end of the compressor. The reduction in the size of the chamber causes the compression of the low pressure gas which is initially trapped in the chamber when the rotation of the rotors closes it off from the suction port. The volume of the chamber decreases and the compression process continues until the chamber is circumferentially displaced to the extent that it comes into communication with the discharge port at the high pressure end of the compressor working chamber.
The chevron shaped chamber formed between the male and female rotors is partially defined, as noted earlier, by the interior walls of the compressor working chamber, including the high pressure end wall thereof. Therefore, the pressure in the chevron shaped chamber acts on the fixed high pressure end wall of the working chamber and increasingly so as the volume of the chevron shaped chamber decreases and the pressure interior of it increases. The effect of the buildup of such pressure is to impose an axial force on the rotor set which biases the rotors away from the high pressure end wall of the working chamber. This is the normal situation which exists during screw compressor operation and such axial thrust on the rotor set toward the suction end of the compressor is accounted for in every screw compressor arrangement. However, while axial thrust on the rotor set toward the low pressure end wall of the working chamber is the norm, axial thrust on the rotor set toward the high pressure end wall is not.
The development of reverse axial thrust on the rotor set of a screw compressor has generally been observed to occur during periods of transient compressor loading, such as during the unloading or shutdown of the compressor. Several untoward results can occur if such reverse axial thrust is allowed to develop on the rotor set and is unaccounted for. The most significant possibility is the contact of the end faces of the rotating screw rotor set with the high pressure end wall of the compressor working chamber during the absence of sufficient lubricant between the surfaces. Such contact can, of course, result in the catastrophic failure of the compressor.
Historically, two primary schools of thought have existed with respect to addressing the problem of reverse axial thrust in a screw compressor. U.S. Pat. No. 4,142,765 to Olsaker teaches that because of reverse thrust considerations in screw compressors, the high pressure end faces of the screw rotors, as well as the high pressure end wall of the working chamber must be fabricated from a surface material such that a material combination is provided which has bearing characteristics. In U.S. Pat. No. 4,185,949, Lundberg merely acknowledges that when the rotors are unloaded in a screw compressor they will tend to be drawn toward the high pressure end wall of the working chamber. Lundberg states that it is assumed that such movement of the rotor set is hindered by the abutting of the high pressure end surfaces of the rotors with the high pressure end wall of the working chamber of the compressor. Once again, it is at least implied that the material selection for and fabrication of the screw rotor set and the rotor housing must be carefully considered in respect of the contemplated contact or abutment of the high pressure end faces of the rotor set with the high pressure end wall of the working chamber of the compressor.
More recently, possibly in recognition of the fact that even a minor and short-lived lubrication failure can cause a catastrophic failure of the compressor where rotor end face to working chamber end wall abutment is an inherent possibility due to a particular screw compressor design, positive steps have been taken in screw compressor bearing arrangements to bias the rotor set in a direction away from the high pressure end wall of the working chamber. Such arrangements generally include the use of a spring or hydraulic pressure to act upon the bearing sets in which the screw rotor shafts are captured at the high pressure end of the compressor in a direction which urges the rotors away from the high pressure end wall of the compressor working chamber.
Exemplary in this regard are U.S. Pat. Nos. 4,227,755 to a second individual named Lundberg and 4,465,446 to Nemit, Jr. et al. In Lundberg '755, yielding means, exemplified both by various springs and a pressure fluid acting on an axially moveable element, act on the bearing sets in which the shafts of the rotors of a screw compressor are captured so as to force the rotor set in a direction toward the low pressure end of the compressor. The various arrangements taught in the '755 patent are such that sufficient force is exerted by the yielding means, under all circumstances, to prevent any axial movement of the rotor set in a direction toward the high pressure end of the compressor. That is, the antifriction bearing of the bearing arrangements taught by the latter Lundberg is at all times kept pressed, by the yielding means disclosed therein, against a fixed surface of the rotor housing.
Nemit, Jr. et al., like the latter Lundberg, teaches the exertion of force upon the bearings of a rotor set which is in a direction toward the low pressure end of the compressor. The force is exerted by a spring which is said to have a load rating sufficient under all circumstances to prevent the further compression of the spring beyond the compression which is occasioned by the compressor assembly process. It is stated that by virtue of this arrangement the clearance established between the high pressure end faces of the rotor set and the high pressure end wall of the working chamber of the compressor is at all times retained as it is established at the time of compressor manufacture.
A disadvantage of the use of biasing springs or the like which have the inherent strength to prevent any movement of the rotor set toward the high pressure end of the compressor is the degree of the frictional forces they impart and which must be overcome as a result of their employment. Such forces are present and must be overcome irrespective of load or thrust conditions whenever the compressor is in operation. As a result of having to continuously carry the increased bearing loads imparted by such arrangements and of having to overcome the friction created by such biasing schemes, bearing life is shortened and compressor energy consumption is increased.